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Long-term borrowings (Details 3) (Long-term debt [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Sep. 12, 2013
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MDU Resources Group, Inc [Member] | Commercial paper revolving credit agreement [Member] |  |
Debt instrument [Line Items] |  |
Ratio of funded debt to capitalization - Company alone, as specified in debt convenants | 65.00% |
Ratio of funded debt to total capitalization as specified in debt convenants | 65.00% |
MDU Energy Capital, LLC [Member] | Master shelf agreement member [Member] |  |
Debt instrument [Line Items] |  |
Ratio of total debt to adjusted total capitalization as specified in debt convenants | 70.00% |
Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants | 65.00% |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |
Ratio - EBIT to interest expense | 150.00% |
MDU Energy Capital, LLC [Member] | Note purchase agreement [Member] |  |
Debt instrument [Line Items] |  |
Weighted average interest rate | 5.30% |
Contracted issuance of senior notes | $ 30.0 |
Intermountain Gas Company [Member] | Revolving credit agreement [Member] |  |
Debt instrument [Line Items] |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |
Centennial Energy Holdings, Inc [Member] | Revolving credit agreement [Member] |  |
Debt instrument [Line Items] |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |
Centennial Energy Holdings, Inc [Member] | Master shelf agreement member [Member] |  |
Debt instrument [Line Items] |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 60.00% |
Ratio of EBITDA to interest expense | 175.00% |
WBI Energy Transmission, Inc. [Member] | Uncommitted long-term private shelf agreement [Member] |  |
Debt instrument [Line Items] |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 55.00% |
Long-term private shelf agreement |  | 175
Notes outstanding | $ 100.0 |

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- Definition Contracted issuance of senior notes. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Long-term private shelf agreement. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Notes outstanding which reduce capacity under the uncommitted private shelf agreement. + References No definition available. + Details
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Balance Type: | credit
Period Type: | instant

X
- Definition Ratio - EBITDA to interest expense. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ratio - EBIT to interest expense. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ratio of funded debt capitalization - Company alone, as specified in debt convenants. + References No definition available. + Details
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Namespace Prefix: | mdu_
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- Definition Ratio of funded debt to total capitalization as specified in debt convenants. + References No definition available. + Details
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- Definition Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants. + References No definition available. + Details
Name: | mdu_RatioOfSubsidiaryDebtToSubsidiaryCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ratio of total debt to adjusted total capitalization as specified in debt convenants. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
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- Definition Ratio of total debt to total capitalization as specified in debt convenants. + References No definition available. + Details
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- Details
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Balance Type: | na
Period Type: | duration

X
- Definition Weighted average interest rate of long-term debt outstanding. + References No definition available. + Details
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